With the increasing need for high-capacity, low-power storage used for multimedia applications, mobile communications and etc., semiconductor device market shared by non-volatile memory, especially flash memory, is becoming larger and larger, and non-volatile memory has acted as a very important type of memory. The primary characteristic of the nonvolatile memory is that it can preserve the stored information for a long period of time without power, having both the characteristic of read-only memory and very high access speed.
The nonvolatile memory on the market today is dominated by flash memory, but the flash devices have disadvantages such as overhigh operating voltages, low speed of operating, not good enough endurance, and short retention time due to the too thin tunnel oxide layer during the device shrinking. Ideal nonvolatile memory should have the conditions of low operating voltage, simple structure, non-destructive reading, fast operation speed, long retention time, good endurance and excellent scalability.
Nonvolatile resistance switching memory (RRAM: resistive switching memory) has caused great concern from the large companies and research institutes of domestic and abroad, due to its advantages of simple device structure (metal-insulator-metal), high device density, low power consumption, fast programming/erase speed, and etc. The resistance switching memory technology is based on that the resistance of the material is capable of reversibly switching between the high resistance state and low resistance state under voltage control. There are a plurality of materials proven to have resistance switching characteristic: (1) organic polymers such as polyimide (PI), AIDCN and CuTCNQ, etc.; (2) multiple metal oxides, such as magneto resistive material like Pr0.7Ca0.3MnO3, La0.7Ca0.3MnO3, doped SrTiO3 or SrZrO3 and etc; (3) binary transition metal oxides such as NiO, Nb2O5, CuOx, ZrO2, HfO2, Ta2O5, TiO2 and etc; (4) solid electrolyte, such as CuS, AgS, AgGeSe and etc.
A sandwich structure based on an easily oxidizable metal/solid electrolyte/inert metal forms a class of important non-volatile resistive switching memory (RRAM, resistive switching random memory), commonly referred to as solid electrolyte based RRAM, programmable metallization cell (PMC), or conductive bridge random access memory (CBRAM). This kind of memories have the advantages of simple structure, fast speed and low power consumption, and it is regarded as one of the strong competitors of the next generation nonvolatile storage technology by the industry. Its working principle is that, under the actuation of applied electric field, the easily anodic oxidizable metal of the metallic upper electrode A (Such as Cu, Ag or Ni, etc.) is oxidized to metal ions A+ under the action of electric field, the metal ions A+ is transferred in the solid electrolyte B under the action of electric field, moving toward the cathode and finally reaching the inert lower electrode C, where it is reduced to metal A. As the metal is continuously deposited at the lower electrode C, the deposited metal finally reaching the upper electrode A, a single or plurality of filamentous metal conductive bridges connecting the upper and lower electrodes are formed, wherein the device resistance is in a low impedance state; under the action of the reverse electric field, the metal conductive bridges are disconnected, the device is restored to a high impedance state. These two resistive states can be converted to each other by the action of the applied electric field.
However, since the process of nucleating and growing of the conductive filament is a random process, the related electrical properties of the device are highly discrete (such as programming voltage and high/low resistance state). If the formation of conductive filaments can be controlled, the uniformity of the electrical parameters of the device will be greatly improved.